Partial coating of platinum group metals on filter for increased soot mass limit and reduced costs

ABSTRACT

A diesel particulate filter has a thin band of washcoated filter material either on the inlet of the upstream side or the outlet of the downstream side. The washcoating is with platinum group metals (PGM), e.g., Pt and Pd added to the surface and pore structure of a DPF. The PDF should provide comparable or improved distance between active regeneration and/or should prevent HC/CO slip during active DPF regeneration.

FIELD OF THE INVENTION

This invention relates generally to motor vehicles, such as trucks, thatare powered by internal combustion engines, particularly diesel enginesthat have exhaust gas treatment devices for treating exhaust gasespassing through their exhaust systems.

BACKGROUND OF THE INVENTION

A known system for treating exhaust gas passing through an exhaustsystem of a diesel engine comprises a diesel oxidation catalyst (DOC)that oxidizes hydrocarbons (HC) to CO2 and H2O and converts NO to NO2 ,and a diesel particulate filter (DPF) that traps diesel particulatematter (DPM). DPM includes soot or carbon, the soluble organic fraction(SOF), and ash (i.e. lube oil additives etc.). The DPF is locateddownstream of the DOC in the exhaust gas flow. The combination of thesetwo exhaust gas treatment devices prevents significant amounts ofpollutants such as hydrocarbons, carbon monoxide, soot, SOF, and ash,from entering the atmosphere. The trapping of DPM by the DPF preventsblack smoke from being emitted from a vehicle's exhaust pipe.

The DOC oxidizes hydrocarbons (HC) and converts NO to NO2 . The organicconstituents of trapped DPM within the DPF, i.e., carbon and SOF, areoxidized within the DPF, using the NO2 generated by the DOC, to form CO2and H2O, which can then exit the exhaust pipe to atmosphere.

The rate at which trapped carbon is oxidized to CO2 is controlled notonly by the concentration of NO2 or O2 but also by temperature.Specifically, there are three important temperature parameters for aDPF.

The first temperature parameter is the oxidation catalyst's “light off”temperature, below which catalyst activity is too low to oxidize HC.Light off temperature is typically around 180-200° C.

The second temperature parameter controls the conversion of NO to NO2 .This NO conversion temperature spans a range of temperatures having botha lower bound and an upper bound, which are defined as the minimumtemperature and the maximum temperature at which 40% or greater NOconversion is achieved. The conversion temperature window defined bythose two bounds extends from approximately 250° C. to approximately450° C.

The third temperature parameter is related to the rate at which carbonis oxidized in the filter. Reference sources in relevant literature callthat temperature the “Balance Point Temperature” (or BPT). It is thetemperature at which the rate of oxidation of particulate, alsosometimes referred to as the rate of DPF regeneration, is equal to therate of accumulation of particulate. The BPT is one of the parametersthat determines the ability of a DPF to enable a diesel engine to meetexpected tailpipe emissions laws and/or regulations.

Typically, a diesel engine runs relatively lean and relatively coolcompared to a gasoline engine. That factor makes natural achievement ofBPT problematic.

Therefore, a DPF requires regeneration from time to time in order tomaintain particulate trapping efficiency. Regeneration involves thepresence of conditions that will burn off trapped particulates whoseunchecked accumulation would otherwise impair DPF effectiveness. While“regeneration” refers to the general process of burning off DPM, twoparticular types of regeneration are recognized by those familiar withthe regeneration technology as presently being applied to motor vehicleengines.

“Passive regeneration” is generally understood to mean regeneration thatcan occur anytime that the engine is operating under conditions thatburn off DPM without initiating a specific regeneration strategyembodied by algorithms in an engine control system. “Activeregeneration” is generally understood to mean regeneration that isinitiated intentionally, either by the engine control system on its owninitiative or by the driver causing the engine control system toinitiate a programmed regeneration strategy, with the goal of elevatingtemperature of exhaust gases entering the DPF to a range suitable forinitiating and maintaining burning of trapped particulates.

Active regeneration may be initiated even before a DPF becomes loadedwith DPM to an extent where regeneration would be mandated by the enginecontrol system on its own. When DPM loading beyond that extent isindicated to the engine control system, the control system forces activeregeneration, and that is sometimes referred to simply as a forcedregeneration.

The creation of conditions for initiating and continuing activeregeneration, whether forced or not, generally involves elevating thetemperature of exhaust gas entering the DPF to a suitably hightemperature.

There are several methods for initiating a forced regeneration of a DPFsuch as retarding the start of main fuel injections or post-injection ofdiesel fuel to elevate exhaust gas temperatures entering the DPF whilestill leaving excess oxygen for burning the trapped particulate matter.Post-injection may be used in conjunction with other procedures and/ordevices for elevating exhaust gas temperature to the relatively hightemperatures needed for active DPF regeneration.

These methods are able to increase the exhaust gas temperaturesufficiently to elevate the catalyst's temperature above catalyst “lightoff” temperature and provide excess HC that can be oxidized by thecatalyst. Such HC oxidation provides the necessary heat to raise thetemperature in the DPF above the BPT.

However, during such short rich operation, the exhaust gas in enrichedwith hydrocarbons (HC) and carbon monoxide (CO) while the oxygenconcentration in the exhaust gas is drastically depleted.

The amount of HC and CO generated by the engine during the richoperation typically exceeds the stoichiometric quantity of NOx that isto be reduced over the catalyst. This excess of reductant, whilenecessary for high NOx reduction efficiencies, leads to HC and CObreakthroughs at the DOC outlet (“HC/Co slip”), wherein the HC/CO slipcannot be oxidized to CO2 and H2O.

Traditional coated DPF have a washcoat throughout the filter to preventHC/CO slip and increased emissions. However, a coated DPF has a lowersoot-mass-limit to prevent deactivation of the catalyst from high bedtemperatures generated during active regeneration.

Uncoated DPF are used in conjunction with a DOC and operated in apassive manner (no active filter regeneration) or if used with an activeregeneration then have a reduced rate of passive regeneration comparedto a coated filter or have an increased risk of HC/CO slip. The timeelapsed between active filter regeneration is decreased as a result ofthe lower rate of passive soot oxidation.

SUMMARY

The disclosed embodiments of the invention provide a DPF which has athin band of washcoated filter material either on the inlet of theupstream side or the outlet of the downstream side. The washcoating iswith platinum group metals (PGM), e.g., Pt and Pd added to the surfaceand pore structure of a DPF. The PDF should provide comparable orimproved distance between active regeneration and/or should preventHC/CO slip during active DPF regeneration. The embodiments of theinvention should also reduce the cost of the after treatment system byminimizing the PGM applied to the filter. The after treatment systemshould operate and function in the same manner as a fully coated DPFalthough the interval between active regeneration could be increased oralternatively the size of the filter can be reduced.

According to a first embodiment, a coating of platinum group metals(PGM), e.g., Pt and Pd is applied to the surface and pore structure of arelatively thin band of filter media within an inlet portion of the DPFmedia on the upstream side. Due to the presence of the coating,additional NO2 can thus be formed and therefore increase the rate ofpassive regeneration. In addition, since the coating is only on a frontportion of the filter, it will be able to burn any HC/CO slip from theDOC during active regeneration while not being exposed to the exothermfrom burning the HC/CO slip coupled with the burning of the storage sooton the filter. It is known that the temperature rise within the DPF isgreater towards the rear of the DPF as opposed to the front. Thisexotherm becomes pronounced in situations where regeneration has beeninitiated but then is quickly interrupted (e.g. drop-to-idle). By havingthe coating only at the front of the filter, it will be possible tominimize HC/CO slip, maintain passive regeneration and increase thesoot-mass-limit of the filter.

According to a second embodiment, a coating of platinum group metals(PGM), e.g., Pt and Pd is applied to the surface and pore structure of arelatively thin band of filter media within an outlet portion of the DPFmedia on the downstream side. Although this will not increase the rateof passive regeneration other than that from the DOC, it will allow forHC/CO slip mitigation during active regeneration. Since the coating ison the downstream side, HC/CO will be in contact with a catalyst even ifit traverses the filter wall toward the inlet. Also, since the coatingwill not be in direct contact with soot, there will not be anyaccelerated soot burn during conditions such as drop-to-idle. This inturn will minimize the peak bed temperature and prevent ring-cracking,pitting or melting which could occur if the coating was on the upstreamside and in contact with the soot. Though this configuration will beexposed to higher peak temperatures than if the coating was placed onthe inlet of the DPF on the upstream side, the impact will not be aspronounced since this configuration is not relying on the coating toperform, enhance or promote passive regeneration. The function of thewashcoat in this configuration is to burn any HC/CO slip during activeregeneration.

Numerous other advantages and features of the present invention will bebecome readily apparent from the following detailed description of theinvention and the embodiments thereof, from the claims and from theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a representative diesel engine andcontrol with an exhaust after-treatment device;

FIG. 2 is a schematic sectional view of a first embodiment DPF of theinvention; and

FIG. 3 is a schematic sectional view of a second embodiment DPF of theinvention.

DETAILED DESCRIPTION

While this invention is susceptible of embodiment in many differentforms, there are shown in the drawings, and will be described herein indetail, specific embodiments thereof with the understanding that thepresent disclosure is to be considered as an exemplification of theprinciples of the invention and is not intended to limit the inventionto the specific embodiments illustrated.

FIG. 1 shows a schematic diagram of an exemplary diesel engine 20 forpowering a motor vehicle. Engine 20 has a processor-based engine controlsystem 22 that processes data from various sources to develop variouscontrol data for controlling various aspects of engine operation. Thedata processed by control system 22 may originate at external sources,such as sensors, and/or be generated internally.

Control system 22 includes an injector driver module 24 for controllingthe operation of electric-actuated fuel injectors 26 that inject fuelinto combustion chambers in the engine cylinder block 28. A respectivefuel injector 26 is associated with each cylinder and comprises a bodythat is mounted on the engine and has a nozzle through which fuel isinjected into the corresponding engine cylinder. A processor of enginecontrol system 22 can process data sufficiently fast to calculate, inreal time, the timing and duration of injector actuation to set both thetiming and the amount of fueling.

Engine 20 further comprises an intake system having an intake manifold30 mounted on block 28. An intercooler 32 and a compressor 34 of aturbocharger 36 are upstream of manifold 30. Compressor 34 draws airthrough intercooler 32 to create charge air that enters each enginecylinder from manifold 30 via a corresponding intake valve that opensand closes at proper times during engine cycles.

Engine 20 also comprises an exhaust system through which exhaust gasescreated by combustion within the engine cylinders can pass from theengine to atmosphere. The exhaust system comprises an exhaust manifold38 mounted on block 28. Exhaust gases pass from each cylinder intomanifold 38 via a respective exhaust valve that opens and closes atproper times during engine cycles.

Turbocharging of engine 20 is accomplished by turbocharger 36 thatfurther comprises a turbine 40 associated with the exhaust system andcoupled via a shaft to compressor 34. Hot exhaust gases acting onturbine 40 cause the turbine to operate compressor 34 to develop thecharge air that provides boost for engine 20.

The exhaust system further comprises a DOC 41 and DPF 42 downstream ofturbine 40 for treating exhaust gas before it passes into the atmospherethrough an exhaust pipe 44. Although the DOC 41 and the DPF 42 are shownas separate components, it is also possible that the DOC 41 and the DPF42 share a common housing.

DPF 42 physically traps a high percentage of DPM in exhaust gas passingthrough it, preventing the trapped DPM from passing into the atmosphere.Oxidation catalyst 46 within the DOC 41 oxidizes hydrocarbons (HC) inthe incoming exhaust gas to CO2 and H2O and converts NO to NO2 . The NO2is then used to reduce the carbon particulates trapped in DPF 42.

With regard to passive and active regeneration as mentioned above, U.S.Pat. No. 6,829,890; and U.S. Published Patent Applications 2008/0184696and 2008/0093153 describe systems and methods for undertakingregeneration. These patents and publications are herein incorporated byreference.

A first embodiment DPF 42 is shown in FIG. 2. The DPF includes a housing47 having an inlet 48 and an outlet 49 and containing a filter media orfilter material throughout. The filter media is composed of (but notlimited to) cordierite, silicon carbide, aluminum titanate, mullite orother porous ceramic material, or woven metal or ceramic fibers.

Ceramic or refractory materials for diesel particulate filters aredescribed in U.S. Pat. Nos. 6,942,708; 4,510,265; and 4,758,272, hereinincorporated by reference.

A washcoat 54 of platinum group metals (PGM), e.g., Pt and Pd is appliedto the surface and pore structure of a relatively thin band 55 of filtermedia 50 within an inlet portion of the DPF media on the upstream sideof the DPF 42. The relatively thin band 55 can be up to about 25% of thelength of the media 50.

Due to the presence of the PGM, additional NO2 can thus be formed andtherefore increase the rate of passive regeneration within the DPF 42.In addition, since the coating 54 is only on a front portion of the DPF42, it will be able to burn any HC/CO slip from the DOC during activeregeneration while not being exposed to the exotherm from burning theHC/CO slip coupled with the burning of the storage soot on the filter.It is known that the temperature rise within the DPF is greater towardsthe rear of the DPF as opposed to the front. This exotherm becomespronounced in situations where regeneration has been initiated but thenis quickly interrupted (e.g. drop-to-idle). By having the coating onlyat the front of the filter, it will be possible to minimize HC/CO slip,maintain passive regeneration and increase the soot-mass-limit of thefilter.

The maximum bed temperature should be limited so that the PGM does notsinter excessively. Also, the interaction between the washcoat andfilter material may lead to lower tolerance to thermal events (peak bedtemperature, axial/radial thermal gradient). In order to preventexcessive PGM sintering or filter failure induced from thewashcoat-filter material interaction, the soot mass limit (SML) can belowered so that an active DPF regeneration event is commanded morefrequently for equivalent volume of filter.

A second embodiment DPF 42 a is shown in FIG. 3. A washcoat 64 ofplatinum group metals (PGM), e.g., Pt and Pd is applied to the surfaceand pore structure of a relatively thin band 65 of filter media 50within an outlet portion of the DPF media on the downstream side of theDPF 42. The relatively thin band 55 can be up to about 25% of the lengthof the media 50. Although this will not increase the rate of passiveregeneration over that generated by the DOC, it will allow for HC/COslip mitigation during active regeneration. Since the coating is on thedownstream side, HC/CO will be in contact with a catalyst even if ittraverses the filter.

Also, since the coating 64 will not be in direct contact with soot,there will not be any accelerated soot burn during conditions such asdrop-to-idle. This in turn should minimize the peak bed temperature andprevent ring-cracking, pitting or melting. Though the coating of thisembodiment will be exposed to higher peak temperatures than if thecoating was placed on the inlet of the DPF on the upstream side, thisshould not be detrimental since this coating is not intended to perform,enhance or promote passive regeneration. The function of the washcoat inthis configuration is to burn any HC/CO slip during active regeneration.

From the foregoing, it will be observed that numerous variations andmodifications may be effected without departing from the spirit andscope of the invention. It is to be understood that no limitation withrespect to the specific apparatus illustrated herein is intended orshould be inferred.

The invention claimed is:
 1. A diesel particulate filter, comprising: ahousing having an inlet and an outlet and containing a dieselparticulate filter material therein wherein a band of said filtermaterial includes a washcoat with platinum group metals added to thesurface and pore structure of the filter material.
 2. The dieselparticulate filter according to claim 1, wherein said band is locatedadjacent to said inlet.
 3. The diesel particulate filter according toclaim 1, wherein said band is located adjacent to said outlet.
 4. Thediesel particulate filter according to claim 1, wherein said platinumgroup metals comprises at least one metal selected from Pt and Pd.
 5. Anexhaust gas after treatment system for a diesel engine, comprising: acontainment defining a flow path for exhaust gas; a diesel oxidationcatalyst unit arranged in the flow path; a diesel particulate filterunit arranged in the flow path downstream of the diesel oxidation unit,wherein the diesel particulate filter comprises a housing having aninlet and an outlet and containing a diesel particulate filter materialtherein wherein a band of said filter material includes a washcoat withplatinum group metals added to the surface and pore structure of thefilter.
 6. The diesel particulate filter according to claim 5, whereinsaid band is located adjacent to said inlet.
 7. The diesel particulatefilter according to claim 6, wherein said platinum group metalscomprises at least one metal selected from Pt and Pd.
 8. The dieselparticulate filter according to claim 5, wherein said band is locatedadjacent to said outlet.
 9. The diesel particulate filter according toclaim 8, wherein said platinum group metals comprises at least one metalselected from Pt and Pd.
 10. The diesel particulate filter according toclaim 5, wherein said platinum group metals comprises at least one metalselected from Pt and Pd.